This proposal will address a number of gaps in the neuropsychological literature that informs best practices for diagnosis and treatment of persistent post-concussive symptoms coupled with impairments in psychological health. Traumatic brain injury (TBI) is often considered the signature injury of Operation Enduring Freedom (OEF) and Operation Iraqi Freedom (OIF). By some estimates, 20% of previously deployed military personnel have sustained TBIs during their service in OEF and OIF, primarily due to blast injuries. The majority of these injuries can be classified as mild TBI or concussion. Furthermore, it is estimated that 14% of Veterans returning from Afghanistan and Iraq have post-traumatic stress disorder (PTSD), and a substantial percentage of Veterans have both conditions. Although most individuals with mild TBI show good recoveries, others can experience lingering deficits in memory, concentration, decision-making, and mood. The frontal lobes of the brain are particularly vulnerable to damage from TBI, and frontal lobe injuries can lead to a number of serious symptoms that can be disabling to the patients and their families. Alterations in attention, concentration, decision-making, personality, and emotion regulation can affect social functioning and employment. However, there is still substantial disagreement about the precise contribution of mild TBI to post- deployment problems. The overlap with PTSD symptoms is extensive, and disentangling the effects of each has been challenging. Furthermore, major depression is highly correlated with the severity of PTSD symptoms, and the frequency of problematic drinking is substantially elevated. Finally, the risk for suicidal ideation is increased in Veterans with PTSD, especially in those with two or more co-morbid disorders. Basic research that considers how all of these variables might interact to produce subtle changes in the brain is lacking. Therefore, this project intends to fill these gaps with a series of experiments that will identify specific deficits in inhibitory control, the ability to withhold inappropriate or ill-conceived responses. Impairments i the brain systems that mediate inhibitory control functions can ultimately lead to impulsive behaviors. Impulsivity is not a unitary construct, however, and separation into different domains will help identify and prioritize those that are the most troublesome for OEF/OIF veterans with mTBI/PTSD. The study will use non-invasive electrophysiological recording of the brain to examine the time course of neural activity underlying three specific inhibitory control functions: (1) motor response inhibition, or the ability to withhold pre-potent motor responses; (2) impulsive choice in the context of reward, or decision making without regard to future consequences; (3) emotion regulation, or the ability to control affective responses. The project will use novel, carefully designed tasks that are sensitive to subtle deficits and relevant to real world functioning, and will focus on different dimensions of impulsivity that can lead to negative psychosocial outcomes. Identifying neurological abnormalities associated with mTBI/PTSD - noninvasively recorded biomarkers of brain function - will further our understanding and inform efforts to develop improved cognitive rehabilitation and other therapeutic interventions. Learning more about the specific anatomical and functional substrates of inhibitory control functions will help in the development of improved diagnosis, treatment, and rehabilitation strategies for Veterans affected by cognitive and emotional dysregulation. Since these conditions can result in tremendous losses of productivity, the development of more sensitive assessments of brain health can assist in tailoring rehabilitation efforts to the needs of individual patients.